GRAIN-BOUNDARY MODIFICATIONS OF MANGANESE FERRITES

The effect of minor additions (<1 wt%) of SiO2 and CaCO3 to manganese ferrites was investigated. Complex impedance analysis showed that the resistivity was modified by altering the electrical response of the grain boundaries. The measured resistivity of the grain boundaries went through a maximum at between 0.2 and 0.4 wt% additions. DC conductivity measurements showed a shift from grain boundary to bulk control of the conductivity at fields > 10(3) V/m. An equivalent circuit model was used to calculate the coefficient of non-ohmic response for Mn ferrites which were less than 3.0 for all compositions investigated. Differential thermal, analysis in a magnetic field showed that Mn ferrite decomposed when subjected to post sintering thermal anneals at temperatures >600 degrees C in air. The addition of less than 1 wt% SiO2 and CaCO3 inhibited this phase decomposition, by preventing the rapid diffusion of oxygen at the grain boundaries. The control of oxidation in ferrites by dopants is in addition to the previously recognized role in the control of the electrical and magnetic properties.